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Attic Air Sealing Guide for Ontario Houses: The Leak List, the Materials, the Order of Work, What It Costs per Square Foot and What to Photograph Before the Insulation Covers It
Insulation & Air Sealing
Air Seal the Attic First: The Holes Are Small, the Water They Move Is Not, and You Only Get to See Them Once
Your ceiling has somewhere between thirty and a hundred holes in it, and you have never seen one. Top plates with a gap where the drywall stops, the plumbing stack, every wire into the attic, the bathroom fan housing, the duct boots, the pot lights, the chimney chase and the hatch. Warm house air goes up through all of them all winter, carrying water that freezes on the underside of the roof. Air sealing is the afternoon spent closing them with foam, caulk and sheet metal before the blower truck arrives. It costs about $1.00 a square foot of attic, $1,200 on a typical house, and it is the part of the attic job that actually changes the heating bill. The insulation is the sweater; this is the windbreaker.
The numbers
$1.00attic air sealing package, per sq ft
$600minimum charge
$45airtight pot light cover, each
$200hatch insulated and weatherstripped
The package rate, $0.75 to $1.50 a square foot of attic floor, covers the top plates, the penetrations, the duct boots, the fan housing and the chase, with the hatch gasket. Pot light covers are $45 each on top because a house might have none or might have thirty. The hatch, insulated and weatherstripped, is $150 to $250; an insulated box over a pull-down stair is $350. A crawl under a low hip roof goes to the top of the range and the installer earns every cent of it.
Why this comes before the insulation and not after
Warm air is lighter than cold, so in January the whole house behaves like a chimney, pushing air out through the ceiling and pulling it in through the basement. The pressure is small, but it runs all day for five months through every hole, and each litre of 21 C house air carries water that a minus 20 C roof deck takes out of it as frost. NRCan’s Keeping the Heat In puts the ceiling at the top of the list for exactly this reason, and says plainly that ventilating the attic harder does not make up for leaving the holes open.
Insulation blown over the holes does not close them. It hides them, and it hides the frost until it melts in March. Once there is 17 in. of cellulose over the top plates nobody is going back to find the gap behind the bathroom wall. So the order is fixed: seal, then baffle, then blow. The owner’s own article on why ICF makes the whole house easier to seal covers the other end of the same chimney.
The leak list
| Leak | Where to find it | What closes it |
|---|---|---|
| Top plates of interior walls | Every wall below, a 1/8 to 1/4 in. gap between the drywall and the plate, both sides, the whole length | One-part foam or acoustical caulk along both edges. The big one; a 2,000 sq ft house has 300 ft of it. |
| Plumbing stack | The 3 or 4 in. ABS pipe through the ceiling, usually with a 1 in. ring around it | Foam around the pipe, or a rubber boot sealed to the drywall. ABS moves with temperature; foam alone can crack. |
| Wires | Every cable up into the attic, each with a hole twice its size | A dab of foam or fire-rated caulk at each. Not over knob-and-tube: that wiring gets replaced before the attic job, not sealed around. |
| Bathroom fan housing | The box in the ceiling, open at the seams and the duct collar | Foil tape and caulk at the housing; the duct insulated and run to a roof or wall hood, never left in the attic. That page. |
| Duct boots | Where a supply duct comes through the ceiling in a bungalow | Mastic or foil tape at the boot to the drywall; the run insulated. |
| Pot lights | Each one a 6 in. hole with a tin can in it, non-IC cans open at the top | An airtight IC-rated cover box over each, $45, or the fixture replaced with an airtight IC LED. The detail. |
| Chimney and flue chase | The framed opening around a masonry chimney or a metal flue, often open to the basement | Sheet metal cut to the chase, caulked with high-temperature sealant, and a 3 in. metal dam to keep insulation off the chimney. Foam does not go here. |
| Dropped ceilings and soffits | Over kitchen cabinets, bathtubs and stair closets; the framing is open to the attic above | Rigid foam or drywall cut to cap the opening, foamed at the edges. The commonest big hole in a 1970s to 1990s house. |
| Electrical boxes in the ceiling | Every light and smoke detector | Foam around the box, not in it; a gasket behind the fixture. |
| The hatch | A square of plywood sitting on trim, no gasket, no insulation | Weatherstripping on the trim, a latch to pull it tight, rigid foam glued to the top, $200. |
The materials, and the two places foam does not belong
Most of the list is one-part polyurethane foam from a gun, which is cheap, fast and sticks to anything dry. Gaps over 1/4 in. get foam; cracks under that get acoustical caulk, the black stuff that never cures hard, or fire-rated caulk at wiring. Big openings, the dropped soffits and the chase, get a lid first, rigid foam, drywall or sheet metal, and then foam at the edges, because foam does not span a hole, it fills one.
Two places foam is wrong. Around a chimney or a gas vent, where the clearance to combustibles applies to the sealant as much as the insulation: sheet metal, high-temperature silicone and the 3 in. dam. And over a non-IC pot light, where the can needs air around it or it cooks; the cover box goes on and the foam seals the box, not the light. An installer foaming around a chimney is the end of the inspection.
Knob-and-tube: if there is any in the attic, the Ontario Electrical Safety Code does not allow insulation in contact with it, most insurers will not carry the house with it buried, and the attic job waits for the electrician. No sealing, no blowing, no exceptions.
The order of work, and the 1,200 sq ft attic priced
- Look first. Vermiculite, wet or mouldy insulation, knob-and-tube, a fan ending in the attic. Any of the first three stops the job until it is dealt with; the inspection checklist is the list.
- Pull the old insulation back from the top plates and penetrations. You cannot seal what you cannot see.
- Seal, from the far end back toward the hatch, with a work light and a mirror for the top plates. A day for two men on 1,200 sq ft.
- Cap and dam: lids on the soffits and chase, the metal dam at the chimney, the pot light covers, a dam at the hatch and at the eave plate.
- Baffles in every rafter bay at the soffit, $3.50 a bay, about $300 on an average house. Attic ventilation covers why.
- Hatch weatherstripped and insulated, depth markers stapled to the trusses, then the blow.
The house I use on every page: 1,200 sq ft attic at R-20 going to R-60. Air sealing 1,200 at $1.00 is $1,200. Baffles $300. Hatch $200. Cellulose top-up 1,200 at $2.60 is $3,120. Subtotal $4,820, HST $627, total $5,447. The sealing is a quarter of the bill and most of the result. Ten pot lights would add $450. If the house qualifies for the standalone attic rebate, existing R-12 to R-25 taken to R-50 or better by a contractor on the program’s participating list, up to $1,000 comes back afterward: 50% of the after-tax cost is $2,723, capped at $1,000. Check the program page before you sign.
What to photograph before it disappears
Every sealed top plate run with the bead visible. The stack with its boot. The fan housing taped and the duct going somewhere. Each pot light cover. The chase lid and the chimney dam with a tape showing 3 in. The soffit lids. The baffles, one photo down the eave line. The hatch gasket. Twenty photos, ten minutes, and they are what a rebate inspector, a home inspector when you sell, and you next February will want to see. The blow covers all of it the same afternoon.
After the blow, a blower door checks all of it: $300 to $450 standalone, or included in an EnerGuide evaluation. A pre-1970 house starts at 8 to 15 air changes an hour at 50 pascals; a good attic seal can take a point or two off that, and the number is the only proof that the foam went where the holes were.
Questions people ask
How much does attic air sealing cost in Ontario?
$0.75 to $1.50 a square foot of attic floor, $1.00 as a default, with a $600 minimum: $1,200 on a 1,200 sq ft attic. Pot light covers are $45 each extra, the hatch $150 to $250, and the soffit baffles about $300.
Should I air seal the attic before adding insulation?
Yes, always. Blown insulation does not stop air; it hides the holes. Seal the top plates, penetrations, fan, chase and hatch, baffle the eaves, then blow. Doing it afterward means digging through 17 in. of insulation to find each gap.
What is the biggest air leak in an attic?
By total length, the top plates of the interior walls, 300 ft of gap in an ordinary house. By single hole, a dropped ceiling over kitchen cabinets or a tub, or a chimney chase open from the basement to the roof.
Can I use spray foam to air seal the attic floor?
Gun foam, yes, at the gaps and penetrations. Not around a chimney or gas vent, where it is sheet metal and high-temperature sealant with a 3 in. clearance, and not over a non-IC pot light, which gets an airtight cover first.
Does air sealing the attic make the house too tight?
Not an existing house with a leaky basement. But a tightened house needs its ventilation looked at: bath fans that work, and if a furnace or water heater draws its air from the room, a check that it still drafts. Ask for that check with the blower door test.
Put the sealing on the quote
The attic insulation cost calculator carries air sealing, pot light covers, the hatch and the baffles as their own lines beside the insulation, so a quote that skipped them shows up as cheap for a reason. If you have quotes already, the $99 review reads them against this leak list and tells you which one is planning to blow over the holes.
Sources and further reading
- Keeping the Heat In, Section 5: Roofs and attics: NRCan on checking existing insulation, fixing moisture first, air sealing before insulating, baffles, hatches, chimneys and pot lights.
- Ontario’s Building Code: the official page with the current regulation; vapour barriers are 9.25, attic ventilation 9.19, thermal barriers over foam 9.10.17, radon 9.13.
- Electrical Safety Authority: Ontario: knob-and-tube wiring, the inspection rules, and why thermal insulation cannot be placed around it.
